Drawing from the worlds of Azeroth, Sanctuary, and the Koprulu Sector, the game currently features 18 heroes. From what we've gathered, the 18 are Abathur, Arthas, Diablo, Elite Tauren Chieftain, Falstad, Gazlowe, Illidan, Kerrigan, Malfurion, Naziba, Nova, Raynor, Sonya, Stitches, Tassadar, Tyrael, Uther, and Valla.

Three of HotS' defining characteristics thus far:

Multiple (4+) interactive battlegrounds, each with an element that can sway the battle.

Team leveling instead of individual leveling, encouraging teams to have an equal skill level.

Customizable ability sets where you can choose which abilities fit your desired role.

The customizable ability sets could allow Blizzard to have fewer playable characters while still maintaining gameplay diversity – both a smart and intriguing move. Team leveling strikes us as potentially controversial; we'll have to see the response during the beta in the first half of 2014.